This invention relates to a noise elimination device capable of eliminating noise by detecting a pulse-like noise mixed in an audio signal by detecting an abnormal change in an original signal on the basis of a linear prediction method and substituting the time portion including the noise by a substituting signal.
There are various noises mixed into an audio signal such, for example, as a noise caused by a code error in a digital signal transmission system, e.g., PCM (pulse code modulation) transmission system and, in an analog transmission system, a clicking noise occurring in playing of a record due to scratches and dirts on the record and an ignition noise occurring in an automobile radio tuner.
Proposals have heretofore been made to eliminate such noise mixed in an audio signal. The process of the noise elimination generally consists of a noise detection process for judging whether an input signal is a normal signal or noise and a signal correction process for restoring an original signal in a case where noise has occurred. For achieving a proper noise elimination, both the noise detection process and the signal correction process must be accurately effected. In the prior art devices, a parity check method utilizing an error detection code as a part of data and a CRC (cyclic redundancy check) code method are generally employed as the noise detection process as is well known in PCM systems. These methods, however, are disadvantageous in that there is limitation in a code format and that they are theoretically not applicable to a noise occurring in an analog signal, i.e., they are applicable only to detection of a noise produced by a burst error or the like cause occurring in transmission of a digitized signal and incapable of detecting a noise contained in the original signal. For overcoming this difficulty, there have been proposed a method for detecting noise on the basis of a prediction error between a linear predicted value of an input signal obtained from successive input signals and an input signal (Sugahara, Ikeda, Yamazaki and Ito: Japan Acoustics Society Lectures and Theses 3-2-6, October 1979) and also a method for detecting noise by utilizing subtraction in the frequency spectrum region (Yoshitani: Electronic Communication Theses 1980/12 Vol. J63A No. 12). These methods, however, are incapable of properly dealing with a uniform change in the noise signal level or tone signals which have little correlation between themselves (e.g. so-called "pink noise" used in a music synthesizer and rising portions of string instrument tones) resulting in an erroneous detection, e.g. overlooking noise or misjudging a musical tone for noise.
As to the signal correction process, prior art methods which have heretofore been generally practiced include a previous data hold method according to which a correct value immediately preceding occurrence of a noise is maintained over a section in which such noise takes places, a linear interpolation method according to which a mean value of a correct value immediately before occurrence of a noise and a correct value immediately after passing of the noise is maintained over a section in which the noise takes place and a method disclosed in the specification of Japanese laid-open patent application No. 55-84010. These methods are found to be effective against a single data error but have no noise suppression effect against a noise occurring over a relatively long period of time of 30 samples to 50 samples (i.e., about one (1) msec at a sampling frequency of 50 kHz) or rather have an adverse effect in that difference in level is produced at a joint between an interpolating signal and an original signal.